To qualitatively and quantitatively identify, serologically, the immunological factors related to resistance and susceptibility of chickens to Marek's disease. It has been demonstrated that families and lines of chickens can be developed by selective breeding for resistance or susceptibility to the disease even though approximately all individuals become infected at an early age under field conditions. It is the purpose of this proposal, therefore, to try to identify the immunological responses associated with competent or defective defense mechanisms of resistant and susceptible chickens respectively and attempt to relate these to the pathogenesis of this infection which more often develops into neoplasia in the susceptible lines. As a model system, this should provide information on the importance of immunological competence or defectiveness to Marek's disease infections and its potential subsequent neoplasia.